linux-kernel - Re: [RFC PATCH] mm: Fetch the dirty bit before we reset the pte

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Message-ID: <2de084c8-9497-7877-56b3-bf1efc615e2e@linux.ibm.com>
Date:   Thu, 8 Oct 2020 23:02:09 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nick Piggin <npiggin@...il.com>, Peter Xu <peterx@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        John Hubbard <jhubbard@...dia.com>,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>, Michal Hocko <mhocko@...e.com>,
        Kirill Shutemov <kirill@...temov.name>,
        Hugh Dickins <hughd@...gle.com>
Subject: Re: [RFC PATCH] mm: Fetch the dirty bit before we reset the pte

On 10/8/20 10:32 PM, Linus Torvalds wrote:
> On Thu, Oct 8, 2020 at 2:27 AM Aneesh Kumar K.V
> <aneesh.kumar@...ux.ibm.com> wrote:
>>
>> In copy_present_page, after we mark the pte non-writable, we should
>> check for previous dirty bit updates and make sure we don't lose the dirty
>> bit on reset.
> 
> No, we'll just remove that entirely.
> 
> Do you have a test-case that shows a problem? I have a patch that I
> was going to delay until 5.10 because I didn't think it mattered in
> practice..
> 

Unfortunately, I don't have a test case. That was observed by code 
inspection while I was fixing syzkaller report.

> The second part of this patch would be to add a sequence count
> protection to fast-GUP pinning, so that GUP and fork() couldn't race,
> but I haven't written that part.
> 
> Here's the first patch anyway. If you actually have a test-case where
> this matters, I guess I need to apply it now..
> 
>                     Linus
> 


-aneesh